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Gallery of GeologyThe Pennsylvanian section at Bishop Cap, Doña Ana County, New Mexico

Spencer G., Lucas, New Mexico Museum of Natural History, 1801 Mountain Road NW, Albuquerque, NM 87104Karl Krainer, Institute of Geology, University of Innsbruck, Innrain 52, Innsbruck A-6020 Austria

Located about 19 km (12 mi) southeast of Las Cruces in Doña Ana County, New Mexico, Bishop Cap is a rugged peak at the southern end of the Organ Mountains. At an elevation of 1,651 m (5,419 ft), Bishop Cap rises about 335 m (1,100 ft) above the alluvial fans at its base. The miter-shaped peak looks like a bishop’s cap, hence the name. Bishop Cap is on a westward-tilted fault block between the Organ Mountains of New Mexico and the Franklin Mountains of West Texas. Its western side is a monocline-like flexure that dips to the west beneath Quaternary alluvial-fan deposits.

The oldest strata exposed at Bishop Cap are along the eastern base of the peak. They are Mississippian marine rocks capped by the Late Mississippian (Chesterian) Helms Formation. A limestone-dominated Pennsylvanian section about 256 m thick makes up most of Bishop Cap, and these strata rest with evident disconformity on olive-gray shale of the Helms Formation.

During the early 20th Century, fluorite was mined at Bishop Cap and in the surrounding hills, and published descriptions of the mining geology and stratigraphy first appeared in the 1920s (Seager, 1973, 1981). The lithostrati-graphic nomenclature long applied to the Pennsylvanian strata at Bishop Cap has been that of Nelson (1940), names he introduced at Vinton Canyon in the northern Franklin Mountains to the south. Nelson (1940) coined the names (ascending order) La Tuna, Berino, and Bishop’s Cap mem-bers of the Magdalena Formation. At Vinton Canyon, these strata and an overlying unnamed interval of Pennsylvanian strata comprise a stratigraphic section about 823 m thick (Harbour, 1972).

At Bishop Cap, Seager (1973, 1981) mapped the Penn-sylvanian strata as the La Tuna and Berino formations of the Magdalena Group (ironically, Nelson’s “Bishop’s Cap Mem-ber” is not exposed at Bishop Cap, having been removed by

The Pennsylvanian stratigraphic section along the southeastern face of Bishop Cap. The slightly folded limestone in the foreground is the base of the Horquilla Formation (“La Tuna Formation,” our unit A) disconformably overlying a shale slope at the top of the Upper Mississippian Helms Formation. Unit A extends up through the lower interval of cliff- and ledge-forming limestone. It is overlain by a slope-forming interval (base of “Berino Formation,” our unit B), followed by a relatively thin cliff of limestone (unit C), another slope-forming interval (unit D) and then the limestones at the top of the peak (unit E).

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erosion). We have measured the Pennsylvanian stratigraphic sections at Bishop Cap and in Vinton Canyon, and they are lithologically very similar to Pennsylvanian strata assigned to the Horquilla Formation to the west (especially in the Big Hatchet Mountains of Hidalgo County: Lucas et al., 2017) and to the northwest in the Robledo Mountains of Doña Ana County (Krainer et al., 2015). Therefore, we assign the Penn-sylvanian section at Bishop Cap to the Horquilla Formation and note that Nelson’s units La Tuna, Berino, and Bishop’s Cap members may prove useful as local, member-level sub-divisions of the Horquilla Formation (Krainer et al., 2020). More study is ongoing to resolve fully the lithostratigraphy and correlation of the Pennsylvanian strata in southern New Mexico and West Texas.

At Bishop Cap, we divide the Pennsylvanian section into five informal units (intervals), A–E. Unit A is the La Tuna Formation as mapped by Seager (1973, 1981). It is ~79 m thick and mostly limestone (79% of the unit thickness), much of it cherty (45% of the thickness). Covered intervals are shale and/or nodular limestone and make up about 20% of the unit thickness. There is a single sandstone bed (bed 20), about 1.1 m thick, with plant fossils of cordaites in it. Most of the limestone beds in unit A are 0.5–2 m thick, cherty

and are as thick as 5.2 m. Mostly they are wackestones and feature silicified fossils of the demisponge Chaetetes (bed 2) and the coral Petalaxis (bed 24). The limestone beds that lack chert or have little chert are mostly crinoidal.

Unit B is ~84 m thick, and it is the lower part of the Berino Formation as mapped by Seager (1973, 1981). This is a slope-forming unit of relatively thick covered intervals (shale and/or nodular limestone) that are more than half (about 54%) of the unit thickness. These covered intervals are intercalated with relatively thin (mostly less than 1 m thick) beds of limestone that are wackestones and some packstones, some cherty and others lacking in chert. Notable are crinoidal packstones that are crossbedded, a bed with abundant fusulinids (bed 110) and a single, 0.5-m-thick bed of limestone-cobble conglomerate.

Unit C is a cliff-forming interval of limestone about 23 m thick. It consists almost entirely of chert-free crinoidal limestone in beds up to 4.2 m thick. Unit D is ~46 m thick and is another slope-forming interval similar to unit B. Thus, unit D is covered intervals (shale and/or nodular limestone) intercalated with thin beds (mostly less than 1 m thick) of crinoidal limestone. Unit E forms the top of Bishop Cap. It is ~20 m thick and is a cliff of limestone beds that are mostly cherty and nodular.

Photo 2. Some details of the Pennsylvanian section at Bishop Cap. Upper left, the silicified demisponge Chatetes in bed 2 of the section; upper right, cordaite plant fossil in bed 20; lower left, the coral Petalaxis in bed 24; lower right, bed of Atokan fusulinids, Fusulinella in bed 110.

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Clopine (1992) documented fusulinids from a section he called Bishop Cap, though he actually sampled in the hills immediately north of the peak. Based on fusulinids, he assigned Morrowan, Atokan and early Desmoinesian ages to the section at Bishop Cap. Conodont biostratigraphy based on samples we collected and under study by James Barrick of Texas Tech University yield ages consistent with Clopine’s fusulinid-based age assignments (Krainer et al., 2020). Thus, the conodonts indicate that the upper part of unit A (~65 m above base) is early Atokan, and the lowest Desmoinesian fauna occurs ~60 m above the base of unit B. Early Desmoi-nesian conodonts range through unit C and as least as high as the lower 10 m of unit D. Less age diagnostic Desmoinesian conodonts occur in the upper part of unit D and unit E. Fur-ther study of the microfossils from the Pennsylvanian strata at Bishop Cap is underway.

In the Horquilla Formation at Bishop Cap, thick-bedded to massive limestone is common in the lower part (units A and C), whereas nodular limestone is absent in unit A, rare in units B, C, and D and common in unit E. Chert is com-mon in units A, B, and E and less common in units C and D. Muddy limestone types (wackestone, packstone, float-stone) containing a diverse fossil assemblage are common throughout the section, indicating deposition in a mostly low-energy, normal marine, open shelf to ramp setting with a slight increase in water depth toward the top (indicated by an increase in nodular limestone). The facies at Bishop Cap show some differences from the Horquilla Formation in the Robledo Mountains where nodular limestone is rare to absent (Krainer et al., 2015). In the Big Hatchet Mountains, the Horquilla Formation is much thicker (1,050 m), and nodular limestone is very rare to absent (Krainer et al., 2017). Thus, the Robledo and Big Hatchet sections of the Horquilla Formation were likely deposited in shallower water than the section at Bishop Cap. For a more detailed reconstruction of the depositional environments a microfacies analysis of the limestone facies is planned.

To understand the regional depositional setting of the Pennsylvanian strata at Bishop Cap, it is important to rec-ognize that during the Early-Middle Pennsylvanian a seaway extended from southeastern Arizona/southwestern New Mexico (Pedregosa basin) across southernmost New Mexico (Orogrande basin, including the location of Bishop Cap) to the western end of the west Texas Delaware basin (e.g., Kues and Giles, 2004, figs. 5, 7, 8). Strata deposited in this seaway are mostly bedded limestones and shales that can locally form shallowing-upward cycles likely driven by glacio-eustasy (Lucas et al., 2017). These are strata of the Horquilla For-mation to the west, grading eastward into strata in southern New Mexico long termed Magdalena Group (Formation or Limestone). We assign these strata in southern New Mexico

to the Horquilla Formation. They were deposited in what we call the Horquilla seaway, an epicontinental sea that was located along part of the southwestern edge of equatorial Pangea during the Pennsylvanian-early Permian.

References

Clopine, W.W., 1992, Lower and Middle Pennsylvanian fusulinid biostratigraphy of southern New Mexico and westernmost Texas: New Mexico Bureau of Mines and Mineral Resources, Bulletin 143, 67 p.

Harbour, R.L., 1972, Geology of the Franklin Mountains, Texas and New Mexico: U. S. Geological Survey, Bulletin 1298, 129 p.

Krainer, K., Lucas, S.G., Vachard, D. and Barrick, J.E., 2015, The Pennsylvanian-Permian section at Robledo Mountain, Doña Ana County, New Mexico, USA: New Mexico Museum of Natural History and Science, Bulletin 65, p. 9–42.

Krainer, K., Lucas, S.G. and Barrick, J.E., 2020, The Penn-sylvanian section at Bishop Cap, Doña Ana County, New Mexico: New Mexico Geology, v. 42, p. 45–46.

Krainer, K., Vachard, D. and Lucas, S.G., 2017, Microfa-cies, microfossils and biostratigraphy of the Pennsyl-vanian-Permian Horquilla Formation, New Well Peak, Big Hatchet Mountains (Hidalgo County, southern New Mexico): New Mexico Museum of Natural History and Science, Bulletin 75, 163 p.

Kues, B.S. and Giles, K.A., 2004, The late Paleozoic ances-tral Rocky Mountains system in New Mexico; in Mack, G.H. and Giles, K.A., eds., The geology of New Mexico: New Mexico Geological Society, Special Publication 11, p. 95–136.

Lucas, S.G., Krainer, K., Barrick, J.E., Vachard, D. and Ritter S.M., 2017, Lithostratigraphy and microfossil biostratig-raphy of the Pennsylvanian-lower Permian Horquilla For-mation at New Well Peak, Big Hatchet Mountains, New Mexico, USA: Stratigraphy, v. 14, p. 233–246.

Nelson, L.A., 1940, Paleozoic stratigraphy of Franklin Mountains, West Texas. American Association of Petro-leum Geologists Bulletin, v. 24, p. 157–172.

Seager, W.R., 1973, Geologic map and sections of Bishop Cap-Organ Mountains area, New Mexico: New Mexico Bureau of Mines and Mineral Resources, Geologic Map 29, scale 1:24,000.

Seager, W.R., 1981, Geology of Organ Mountains and south-ern San Andres Mountains, New Mexico: New Mexico Bureau of Mines and Mineral Resources, Memoir 36, 97 p.